Fuel dilution for reducing NOx production

ABSTRACT

A combustion device and combustion method for mixing a fuel and a fluid to form a diluted fuel mixture and passing the diluted fuel mixture through a nozzle. The nozzle comprises a nozzle body having an inlet face, an outlet face, and an inlet flow axis passing through the inlet face and the outlet face, and one or more slots extending through the nozzle body from the inlet face to the outlet face, each slot having a slot axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/713,232, filed Nov. 14, 2003 and also a continuation-in-partof U.S. patent application Ser. No. 10/786,281, filed Feb. 25, 2004,which is a Division of U.S. patent application Ser. No. 10/353,683,filed Jan. 29, 2003, U.S. Pat. No. 6,866,503, each incorporated hereinby reference.

BACKGROUND

Nozzles are used in a wide variety of applications to inject one fluidinto another fluid and promote efficient mixing of the two fluids. Suchapplications include, for example, chemical reactor systems, industrialburners in process furnaces, fuel injectors in gas turbine combustors,jet engine exhaust nozzles, fuel injectors in internal combustionengines, and chemical or gas injection in wastewater treatment systems.Industrial burners may be used in heating reformers, process heaters,boilers, ethylene crackers, or other high temperature furnaces. Theobjective in these applications is to promote vortical mixing and rapiddispersion of the injected fluid into the surrounding fluid. It isusually desirable to achieve this efficient mixing with a minimumpressure drop of the injected fluid.

The proper design of injection nozzles for burners in industrialfurnaces and boilers is important for maximizing combustion efficiencyand minimizing the emissions of carbon monoxide and oxides of nitrogen(NO_(x)). In particular, tightening regulations on NO_(x) emissions willrequire improved and highly efficient nozzle and burner designs for alltypes of fuels used in industrial furnaces and boilers. Burners in thesecombustion applications utilize fuels such as natural gas, propane,hydrogen, refinery offgas, and other fuel gas combinations of varyingcalorific values. Air, preheated air, gas turbine exhaust,oxygen-depleted air, industrial oxygen, and/or oxygen-enriched air canbe used as oxidants in the burners.

Conventional turbulent jets can be used in a circular nozzle tip toentrain secondary or surrounding combustion gases in a furnace by atypical jet entrainment process. The entrainment efficiency can beaffected by many variables including the primary fuel and oxidantinjection velocity or supply pressure, secondary or surrounding fluidflow velocity, gas buoyancy, primary and secondary fluid density ratio,and the fuel nozzle design geometry. Efficient low NO_(x) burner designsrequire nozzle tip geometries that yield maximum entrainment efficiencyat a given firing rate or at given fuel and oxidant supply pressures.Higher entrainment of furnace gases followed by rapid mixing betweenfuel, oxidant gas, and furnace gases produce lower average flametemperatures, which reduce thermal NO_(x) formation rates. Enhancedmixing in the furnace space also can reduce CO levels in the flue gas.If the nozzle design geometry is not optimized, the nozzle may requiremuch higher fuel and/or oxidant supply pressures or higher average gasvelocities to achieve proper mixing in the furnace and yield therequired NO_(x) emission levels.

In many processes in the chemical industry, the fuel supply pressure islimited due to upstream or downstream processes. For example, in theproduction of hydrogen or synthesis gas from natural gas by steammethane reforming (SMR), a reformer reactor furnace fired by a primarynatural gas fuel produces a raw synthesis gas stream. After optionalwater gas shift to maximize conversion to hydrogen, a pressure swingadsorption (PSA) system is used to recover the desired product from thereformer outlet gas. Combustible waste gas from the PSA system,so-called PSA offgas, which typically is recovered at a low pressure, isrecycled to the reformer as additional or secondary fuel. High productrecovery and separation efficiency in a PSA system requires thatblowdown and purge steps occur at pressures approaching atmospheric, andtypically these pressures are as low as practical to maximize productrecovery. Therefore, most PSA systems typically produce a waste gasstream at 5 to 8 psig (135 kPa to 155 kPa) for recycle to the reformerfurnace. After a surge tank to even out cyclic pressure fluctuations andnecessary flow control equipment for firing control, the waste gassupply pressure available for secondary fuel to the reformer furnaceburners may be less than 3 psig (120 kPa).

For cost-effective control of NO_(x) emissions from SMR processfurnaces, the burners should be capable of firing at these low secondaryfuel supply pressures. If the burners cannot operate at these lowpressures, the secondary fuel must be compressed, typically usingelectrically-driven compressors. For large hydrogen plants, the cost ofthis compression can be a significant portion of the overall operatingcost, and it is therefore desirable to operate the reformer furnaceburners directly on low-pressure PSA waste gas as the secondary fuel.

Some commercially-available low NO_(x) burners use active mixing controlmethods such as motor-driven vibrating nozzle flaps or solenoid-drivenoscillating valves to produce fuel-rich and/or fuel-lean oscillatingcombustion zones in the flame region. In these burners, external energyis used to increase turbulent intensity of the fuel and oxidant jets toimprove mixing rates. However, these methods cannot be used in all lowNO_(x) burner designs or heating applications because of furnace spaceand flame envelope considerations. Other common NO_(x) control methodsinclude dilution of fuel gas with recirculated flue gas or the injectionof steam. By injecting non-reactive or inert chemical species in thefuel-oxidant mixture, the average flame temperature is reduced and thusNO_(x) emissions are reduced. However, these methods require additionalpiping and costs associated with transport of flue gas, steam, or otherinert gases. In addition, there is an energy penalty due to the requiredheating of dilution gases from ambient temperature to the processtemperature.

It is desirable that new low NOx burner designs utilize cost-effectivepassive mixing techniques to improve process economics. Such passivetechniques utilize internal fluid energy to enhance mixing and requireno devices that use external energy. In addition, new low NO_(x) burnersshould be designed to operate at very low fuel gas pressures.Embodiments of the present invention, which are described below anddefined by the claims which follow, present improved nozzle and burnerdesigns which reduce NO_(x) emissions to very low levels while allowingthe use of very low pressure fuel gas.

BRIEF SUMMARY

In various embodiments, the invention relates to a nozzle comprising anozzle body having an inlet face, an outlet face, and an inlet flow axispassing through the inlet face and the outlet face, and two or moreslots extending through the nozzle body from the inlet face to theoutlet face, each slot having a slot axis. The slot axis of at least oneof the slots is not parallel to the inlet flow axis of the nozzle body.The nozzle may further comprise a nozzle inlet pipe having a first endand a second end, wherein the first end is attached to and in fluid flowcommunication with the inlet face of the nozzle body. The slot axes ofat least two slots in the nozzle may or may not be parallel to eachother. The ratio of the axial slot length to the slot height may bebetween about 1 and about 20.

At least two of the slots in the nozzle may intersect each other. Thenozzle may have three or more slots and one of the slots may beintersected by each of the other slots. In one configuration, the nozzlehas four slots wherein a first and a second slot intersect each otherand a third and a fourth slot intersect each other.

In various embodiments, the invention relates to a nozzle comprising anozzle body having an inlet face, an outlet face, and an inlet flow axispassing through the inlet face and the outlet face, and two or moreslots extending through the nozzle body from the inlet face to theoutlet face, each slot having a slot axis and a slot center plane. Noneof the slots intersect other slots and all of the slots are in fluidflow communication with a common fluid supply conduit. The center planeof at least one slot may intersect the inlet flow axis.

In various embodiments, the invention relates to a nozzle comprising anozzle body having an inlet face, an outlet face, and an inlet flow axispassing through the inlet face and the outlet face, and two or moreslots extending through the nozzle body from the inlet face to theoutlet face, each slot having a slot axis and a slot center plane. Afirst slot of the two or more slots may be intersected by each of theother slots and the slot center plane of at least one of the slots mayintersect the inlet flow axis of the nozzle body. The center plane ofthe first slot may intersect the inlet flow axis at an included angle ofbetween 0 and about 30 degrees. The center plane of any of the otherslots may intersect the inlet flow axis at an included angle of between0 and about 30 degrees. The center planes of two adjacent other slotsmay intersect at an included angle of between 0 and about 15 degrees.The two adjacent other slots may intersect at the inlet face of thenozzle body.

In various embodiments, the invention relates to a burner comprising:

-   -   (a) a central flame holder having inlet means for an oxidant        gas, inlet means for a primary fuel, a combustion region for        combusting the oxidant gas and the primary fuel, and an outlet        for discharging a primary effluent from the flame holder; and    -   (b) a plurality of secondary fuel injector nozzles surrounding        the outlet of the central flame holder, wherein each secondary        fuel injector nozzle comprises        -   (1) a nozzle body having an inlet face, an outlet face, and            an inlet flow axis passing through the inlet face and the            outlet face; and        -   (2) one or more slots extending through the nozzle body from            the inlet face to the outlet face, each slot having a slot            axis and a slot center plane.

Each secondary fuel injector nozzle of the burner assembly may have twoor more slots and the slot axes of at least two slots may not beparallel to each other. Each secondary fuel injector nozzle may have twoor more slots and at least two of the slots may intersect each other.The nozzle body may have four slots, wherein a first and a second slotintersect each other, and wherein a third and a fourth slot intersecteach other.

Alternatively, the nozzle body may have three or more slots and a firstslot may be intersected by each of the other slots. The center plane ofthe first slot may intersect the inlet flow axis at an included angle ofbetween 0 and about 15 degrees. The center plane of any of the otherslots may intersect the inlet flow axis at an included angle of between0 and about 30 degrees. The center planes of two adjacent other slotsmay intersect at an included angle of between 0 and about 15 degrees.The two adjacent slots may intersect at the inlet face of the nozzlebody.

In various embodiments, the invention relates to a combustion processcomprising:

-   -   (a) providing burner assembly including:        -   (1) a central flame holder having inlet means for an oxidant            gas, inlet means for a primary fuel, a combustion region for            combusting the oxidant gas and the primary fuel, and an            outlet for discharging a primary effluent from the flame            holder; and        -   (2) a plurality of secondary fuel injector nozzles            surrounding the outlet of the central flame holder, wherein            each secondary fuel injector nozzle comprises            -   (2a) a nozzle body having an inlet face, an outlet face,                and an inlet flow axis passing through the inlet face                and the outlet face; and            -   (2b) one or more slots extending through the nozzle body                from the inlet face to the outlet face, each slot having                a slot axis and a slot center plane;    -   (b) introducing the primary fuel and the oxidant gas into the        central flame holder, combusting the primary fuel with a portion        of the oxidant gas in the combustion region of the flame holder,        and discharging a primary effluent containing combustion        products and excess oxidant gas from the outlet of the flame        holder; and    -   (c) injecting the secondary fuel through the secondary fuel        injector nozzles into the primary effluent from the outlet of        the flame holder and combusting the secondary fuel with excess        oxidant gas.

The primary fuel and the secondary fuel may be gases having differentcompositions. The primary fuel may be natural gas and the secondary fuelmay comprise hydrogen, methane, carbon monoxide, and carbon dioxideobtained from a pressure swing adsorption system. The secondary fuel maybe introduced into the secondary fuel injector nozzles at a pressure ofless than about 3 psig (122 kPa). The primary fuel and the secondaryfuel may be gases having the same compositions.

As defined herein, an oxidant gas is an oxygen-containing gas, forexample air, oxygen-depleted air, oxygen-enriched air, and industrialoxygen.

In various embodiments, the invention relates to a combustion methodcomprising:

-   -   (a) mixing a first substantially gaseous fuel having a first        fuel index and a fluid having a second fuel index which is        different from the first fuel index in a conduit thereby forming        a diluted fuel mixture; and    -   (b) passing the diluted fuel mixture through a nozzle, the        nozzle comprising:        -   (1) a nozzle body having an inlet face, and outlet face, and            an inlet flow axis passing through the inlet face and the            outlet face; and        -   (2) one or more slots extending through the nozzle body from            the inlet face to the outlet face, each slot having a slot            axis.

A residence time for the diluted fuel mixture in the conduit is definedas the volume of the conduit divided by the volumetric flow rate of thecombined fuel and fluid streams. The residence time may be 0.1 to 10milliseconds.

The nozzle may comprise two or more slots extending through the nozzlebody from the inlet face to the outlet face. The second fuel index maybe less than the first fuel index by at least by at least 0.1, or by atleast 0.25, or by at least 0.75.

The fluid may be a second substantially gaseous fuel which is differentthan the first substantially gaseous fuel. The fluid may comprise fluegas. The fluid may comprise hydrogen PSA offgas. The fluid may compriseor be steam, carbon dioxide, nitrogen, argon, helium, xenon, krypton ormixtures thereof. The first substantially gaseous fuel may comprise orbe refinery offgas, natural gas, hydrogen PSA offgas, methane, propaneor mixtures thereof.

As defined herein, a substantially gaseous fuel is a fuel that contains0 to 10% by weight solid and/or liquid. According to this definition, asubstantially gaseous fuel may be a completely gaseous fuel i.e. 0% byweight solid and/or liquid. A substantially gaseous fuel will generallynot require atomization.

The combustion method may further comprise:

-   -   (c) introducing an oxidant gas; and    -   (d) combusting at least a portion of the diluted fuel mixture        with at least a portion of the oxidant gas.

Alternatively, the combustion method may further comprise:

-   -   (e) entraining a furnace gas in at least a portion of the        diluted fuel mixture in a furnace thereby forming a furnace gas        entrained fuel mixture;    -   (c) introducing an oxidant gas; and    -   (d′) combusting at least a portion of the furnace gas entrained        fuel mixture with at least a portion of the oxidant gas.

The combustion method may further comprise:

-   -   (f) swirling at least one of the first substantially gaseous        fuel and the fluid prior to mixing the first substantially        gaseous fuel and the fluid.

As used herein, swirling has its conventional meaning as used in thefield of combustion.

The combustion method may further comprise

-   -   (g) providing the nozzle.

In various embodiments, the invention relates to a combustion devicecomprising:

-   -   (a) a first conduit portion for conveying a fuel;    -   (b) a second conduit portion for conveying a fluid which is        different from the fuel;    -   (c) a mixing conduit in fluid communication with the first        conduit portion and in fluid communication with the second        conduit portion for mixing the fuel and the fluid to form a        diluted fuel mixture; and    -   (d) a nozzle in fluid communication with the mixing conduit for        passing the diluted fuel mixture therethrough, the nozzle        comprising:        -   (1) a nozzle body having an inlet face, an outlet face, and            an inlet flow axis passing through the inlet face and the            outlet face; and        -   (2) one or more slots defined by and extending through the            nozzle body from the inlet face to the outlet face, each            slot having a slot axis.

The nozzle of the combustion device may comprise two or more slotsextending through the nozzle body from the inlet face to the outletface. The slot axis of at least one of the slots may not be parallel tothe inlet flow axis of the nozzle body. The slot axes of at least two ofthe slots may not be parallel to each other. At least two of the slotsmay intersect each other or none of the slots may intersect.

The nozzle of the combustion device may have three or more slots whereina first slot of the three or more slots intersects with a second slot ofthe three or more slots and a third slot of the three or more slots. Thenozzle of the combustion device may have four or more slots wherein afirst slot and a second slot intersect each other and a third slot and afourth slot intersect each other.

The first conduit portion may be disposed within the second conduitportion. The length of the mixing conduit may be 2 to 20 times theequivalent diameter of the first conduit portion outlet. Alternatively,the second conduit portion may be disposed within the first conduitportion and the length of the mixing conduit may be 2 to 20 times theequivalent diameter of the second conduit portion outlet. Alternatively,the first conduit portion is not be disposed within the second conduitportion and the second conduit portion is not be disposed within thefirst conduit portion.

The combustion device may further comprise a swirling means in at leastone of the first conduit portion and the second conduit portion.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention are illustrated by the followingdrawings, which are not necessarily to scale.

FIG. 1 is an isometric view of an exemplary nozzle assembly and nozzlebody.

FIG. 2 is an axial section drawing of the nozzle body of FIG. 1.

FIG. 3A is a front perspective view of the tip of the nozzle body ofFIG. 1.

FIG. 3B is a top sectional view of the nozzle body of FIG. 1.

FIG. 3C is a side sectional view of the nozzle body of FIG. 1.

FIG. 3D is a rear view of the tip of the nozzle body of FIG. 1.

FIG. 4 is an isometric drawing of another exemplary nozzle assembly andnozzle body.

FIG. 5A is a front perspective view of the nozzle body of FIG. 4.

FIG. 5B is a side sectional view of the nozzle body of FIG. 4.

FIG. 5C is a top sectional view of the nozzle body of FIG. 4.

FIGS. 6A to 6F are schematic front views of several nozzle variations.

FIGS. 7A to 7F are schematic front views of several alternative nozzlevariations.

FIG. 8 is a schematic view of a burner assembly utilizing secondarynozzles according to an embodiment of the invention.

FIG. 9 is a schematic front view of the burner assembly of FIG. 8.

FIGS. 10A to 10C show representative top and side sectional views and afront view of a burner staging nozzle with circular injector holes.

FIG. 11 shows dimension notation of the nozzle of FIGS. 4, 5A, 5B, and5C.

FIG. 12 shows dimension notation of the nozzle of FIGS. 1, 2, 3A, 3B,3C, and 3D.

FIG. 13 is a plot of fuel pressure vs. firing rate for burnerembodiments of the invention compared with the circular nozzle of FIGS.10A to 10C.

FIG. 14 is a plot of NOx emission concentration vs. firing rate forburner embodiments of the invention compared with the circular nozzle ofFIGS. 10A to 10C.

FIG. 15 is an axial section drawing of a nozzle assembly.

FIG. 16 is an axial section drawing of a nozzle assembly.

FIG. 17 is an axial section drawing of a nozzle assembly.

DETAILED DESCRIPTION

Various embodiments of the present invention include a nozzle or fluidinjection device for the introduction of a primary fluid into asecondary fluid to promote the efficient mixing of the two fluids.Embodiments of the nozzle are characterized by the use of oriented slotsfor injecting the primary fluid and promoting rapid vortical mixing withthe secondary fluid by flow-induced downstream instabilities and a highlevel of small-scale and molecular mixing between the two fluids. Themixing may be achieved rapidly in a short axial distance from the nozzleoutlet. Embodiments of the nozzle may be used in numerous applicationsincluding, for example, chemical reactor systems, industrial burners inprocess furnaces, fuel injectors in gas turbine combustors, jet engineexhaust nozzles, fuel injectors in internal combustion engines, andchemical or gas injection in wastewater treatment systems. The nozzlesare particularly useful for the rapid mixing of fuel, oxidant, andcombustion gases in process furnaces, boilers, and other combustionsystems.

An exemplary embodiment of the invention is illustrated in FIG. 1.Nozzle assembly 1 comprises nozzle body 3 joined to nozzle inlet pipe 5.Slot 7, illustrated here as vertically-oriented, is intersected by slots9, 11, 13, and 15. The slots are disposed between outlet face 17 and aninlet face (not seen) at the connection between nozzle body 3 and nozzleinlet pipe 5. Fluid 19 flows through nozzle inlet pipe 5 and throughslots 7, 9, 11, 13, and 15, and then mixes with another fluidsurrounding the slot outlets. In addition to the slot pattern shown inFIG. 1, other slot patterns are possible as described later; the nozzleassembly can be used in any orientation and is not limited to thegenerally horizontal orientation shown. When viewed in a directionperpendicular to outlet face 17, exemplary slots 9, 11, 13, and 15intersect slot 7 at right angles. Other angles of intersection arepossible between exemplary slots 9, 11, 13, and 15 and slot 7. Whenviewed in a direction perpendicular to outlet face 17, exemplary slots9, 11, 13, and 15 are parallel to one another; however, otherembodiments are possible in which one or more of these slots are notparallel to the remaining slots.

The term “slot” as used herein is defined as an opening through a nozzlebody or other solid material wherein any slot cross-section (i.e., asection perpendicular to the inlet flow axis defined below) isnon-circular and is characterized by a major axis and a minor axis. Themajor axis is longer than the minor axis and the two axes are generallyperpendicular. For example, the major cross-section axis of any slot inFIG. 1 extends between the two ends of the slot cross-section; the minorcross-section axis is perpendicular to the major axis and extendsbetween the sides of the slot cross-section. The slot may have across-section of any non-circular shape and each cross-section may becharacterized by a center point or centroid, where centroid has theusual geometric definition.

A slot may be further characterized by a slot axis defined as a lineconnecting the centroids of all cross-sections of a slot. In addition, aslot may be characterized or defined by a center plane which intersectsthe major cross-section axes of all cross-sections of a slot. Each slotcross-section may have perpendicular symmetry on either side of thiscenter plane. The center plane extends beyond either end of the slot andmay be used to define the slot orientation relative to the nozzle bodyinlet flow axis as described below.

Axial section I-I of the nozzle of FIG. 1 is given in FIG. 2. Inlet flowaxis 201 passes through the center of nozzle inlet pipe 5, inlet face203, and outlet face 17. In this embodiment, the center planes of slots9, 11, 13, and 15 lie at angles to inlet flow axis 201 such that fluidflows from the slots at outlet face 17 in diverging directions frominlet flow axis 201. The center plane of slot 7 (only a portion of thisslot is seen in FIG. 2) also lies at an angle to inlet flow axis 201. Aswill be seen later, this exemplary feature directs fluid from the nozzleoutlet face in another diverging direction from inlet flow axis 201. Inthis exemplary embodiment, when viewed in a direction perpendicular tothe axial section of FIG. 2, slots 9 and 11 intersect at inlet face 203to form sharp edge 205, slots 11 and 13 intersect to form sharp edge207, and slots 13 and 15 intersect to from sharp edge 209. These sharpedges provide aerodynamic flow separation to the slots and reducepressure drop associated with bluff bodies. Alternatively, these slotsmay intersect at an axial location between inlet face 203 and outletface 17, and the sharp edges would be formed within nozzle body 3.Alternatively, these slots may not intersect when viewed in a directionperpendicular to the axial section of FIG. 2, and no sharp edges wouldbe formed.

The term “inlet flow axis” as used herein is an axis defined by the flowdirection of fluid entering the nozzle at the inlet face, wherein thisaxis passes through the inlet face and the outlet face. Typically, butnot in all cases, the inlet flow axis is perpendicular to the center ofnozzle inlet face 205 and/or outlet nozzle face 17, and meets the facesperpendicularly. When nozzle inlet pipe 5 is a typical cylindricalconduit as shown, the inlet flow axis may be parallel to or coincidentwith the conduit axis.

The axial slot length is defined as the length of a slot between thenozzle inlet face and outlet face, for example, between inlet face 203and outlet face 17 of FIG. 2. The slot height is defined as theperpendicular distance between the slot walls at the minor cross-sectionaxis. In case another slot intersects the given slot along the minorcross-section axis, the slot height is the effective height as if theintersecting slot were not there. The ratio of the axial slot length tothe slot height may be between about 1 and about 20.

The multiple slots in a nozzle body may intersect in a planeperpendicular to the inlet flow axis. As shown in FIG. 1, for example,slots 9, 11, 13, and 15 intersect slot 7 at right angles. If desired,these slots may intersect in a plane perpendicular to the inlet flowaxis at angles other than right angles. Adjacent slots also mayintersect when viewed in a plane parallel to the inlet flow axis, i.e.,the section plane of FIG. 2. As shown in FIG. 2, for example, slots 9and 11 intersect at inlet face 203 to form sharp edge 205 as earlierdescribed. The angular relationships among the center planes of theslots, and also between the center plane of each slot and the inlet flowaxis, may be varied as desired. This allows fluid to be discharged fromthe nozzle in any selected direction relative to the nozzle axis.

Additional views of exemplary nozzle body 3 are given in FIGS. 3A to 3D.FIG. 3A is a front perspective view of the nozzle body; FIG. 3B is aview of section II-II of FIG. 3A and illustrates the angles formedbetween the center planes of the slots and the inlet flow axis. Angle α₁is formed between the center plane of slot 15 and inlet flow axis 201and angle α₂ is formed between the center plane of slot 9 and inlet flowaxis 201. Angles α₁ and α₂ may be the same or different, and may be inthe range of 0 to about 30 degrees. Angle α₃ is formed between thecenter plane of slot 11 and inlet flow axis 201 and angle α₄ is formedbetween the center plane of slot 13 and inlet flow axis 201. Angles α₃and α₄ may be the same or different, and may be in the range of 0 toabout 30 degrees. The center planes of any two adjacent other slots mayintersect at an included angle of between 0 and about 15 degrees.

FIG. 3C is a view of section III-III of FIG. 3A which illustrates theangle β₁ formed between the center plane of slot 7 and inlet flow axis201. Angle β₁ may be in the range of 0 to about 30 degrees. The outeredges of slot 11 (as well as slots 9, 13, and 15) may be parallel to thecenter plane of slot 7.

FIG. 3D is a rear perspective drawing of the nozzle body of FIG. 1 whichgives another view of sharp edges 205, 207, and 209 formed by theintersections of slots 9, 11, 13, and 15.

Another embodiment of the invention is illustrated in FIG. 4 in whichthe slots in nozzle body 401 are disposed in the form of two crosses 403and 405. A front perspective view of the nozzle body is shown in FIG. 5Ain which cross 403 is formed by slots 507 and 509 and cross 405 isformed by slots 511 and 513. A view of section IV-IV of FIG. 5A showsthe center planes of slots 509 and 511 diverging from inlet flow axis515 by angles α₅ and α₆. Angles α₅ and α₆ may be the same or differentand may be in the range of 0 to about 30 degrees. The outer edges ofslot 507 may be parallel to the center plane of slot 509 and the outeredges of slot 513 may be parallel to the center plane of slot 511. Inthis embodiment, slots 507 and 511 intersect to form sharp edge 512.

A view of section V-V of FIG. 5A is shown in FIG. 5C, which illustrateshow the center plane of slot 513 diverges from inlet flow axis 515 byincluded angle β₂, which may be in the range of 0 to about 30 degrees.The outer edges of slot 511 may be parallel to the center plane of slot513.

As described above, slots may intersect other slots in either or both oftwo configurations. First, slots may intersect when seen in a viewperpendicular to the nozzle body outlet face (see, for example, FIG. 3Aor 5A) or when seen in a slot cross-section (i.e., a sectionperpendicular to the inlet flow axis between the inlet face and outletface). Second, adjacent slots may intersect when viewed in a sectiontaken parallel to the inlet flow axis (see, for example, FIGS. 2, 3B,and 5B). An intersection of two slots occurs by definition when a planetangent to a wall of a slot intersects a plane tangent to a wall of anadjacent slot such that the intersection of the two planes lies betweenthe nozzle inlet face and outlet face, at the inlet face, and/or at theoutlet face. For example, in FIG. 2, a plane tangent to a wall of slot 9intersects a plane tangent to a wall of slot 7 and the intersection ofthe two planes lies between inlet face 203 and outlet face 17. A planetangent to upper wall of slot 9 and a plane tangent to the lower wall ofslot 11 intersect at edge 205 at inlet face 203. In another example, inFIG. 5B, a plane tangent to the upper wall of slot 513 and a planetangent to the lower wall of slot 507 intersect at edge 512 between thetwo faces of the nozzle.

Each of the slots in the exemplary embodiments described above hasgenerally planar and parallel internal walls. Other embodiments arepossible in which the planar walls of a slot may converge or divergerelative to one another in the direction of fluid flow. In otherembodiments, the slot walls may be curved rather than planar.

Each of the slots in the exemplary embodiments described above has agenerally rectangular cross-section with straight sides and curved ends.Other embodiments using slots with other cross-sectional shapes arepossible as illustrated in FIGS. 6A to 6F. FIGS. 6A, 6B, and 6C showexemplary configurations with intersecting slots having oval,triangular, and rectangular cross-sections, respectively, as seen in afront view of the outlet face of a nozzle body. FIGS. 6D, E, and F showexemplary configurations with multiple intersecting slots havingrectangular, spike-shaped, and flattened oval shapes, respectively, asseen in a front view of the outlet face of a nozzle body.

Other configurations of intersecting slots can be envisioned which fallwithin the scope of the invention as long as each slot has anon-circular cross-section and can be characterized by a slot axis and aslot center plane as defined above. For example, two slots may intersectat the ends in a chevron-shaped or V-shaped configuration. Multipleslots may form multiple intersecting chevrons in a saw-toothed orzig-zag configuration.

In the embodiments described above with reference to FIGS. 1 to 6, thenozzle openings are formed by multiple slots that intersect when seen ina front view of the outlet face of the nozzle body (for example, seeFIG. 3A). Alternative embodiments of the invention are possible in whichmultiple slots do not intersect when seen in a front view of the outletface of the nozzle body. Several of these embodiments are illustrated bythe nozzle body outlet face views of slots in FIGS. 7A through 7F, whichshow separate multiple slots having flattened oval, triangular,rectangular, and spike-shaped cross-sections. The center planes of oneor more of these slots may be parallel to the nozzle body inlet flowaxis; alternatively, the center planes of one or more of these slots mayintersect the nozzle body inlet flow axis. Some of these slots mayintersect one another when viewed in a section parallel to the inletflow axis in a manner analogous to the slots of FIG. 3B. In theembodiments of FIGS. 7A to 7F, the fluid supply to all slots typicallyis provided from a common fluid supply conduit or plenum.

Many of the applications of the nozzles described above may utilize anozzle body which is joined axially to a cylindrical pipe as illustratedin FIGS. 1 through 5. Other applications are possible, for example, inwhich multiple nozzle bodies are installed in the walls of a manifold orplenum which provides a common fluid supply to the nozzle bodies. It isalso possible, and is considered an embodiment of the invention, tofabricate an integrated nozzle manifold or plenum in which the nozzleslots are cut directly into the manifold or plenum walls. In such anembodiment, the role of the nozzle bodies as described above would beprovided by the section of manifold wall surrounding a group of slotswhich forms an individual nozzle.

The slotted nozzles described above provide a high degree of mixingutilizing novel nozzle tip geometries having multiple or intersectingslots which create intense three-dimensional axial and circumferentialvortices or vortical structures. The interaction of these vortices withjet instabilities causes rapid mixing between the primary and secondaryfluids. Mixing can be achieved at relatively low injected fluid pressuredrop and can be completed in a relatively short axial distance from thenozzle discharge. The use of these slotted nozzles provides analternative to active mixing control methods such as boosting the fluidsupply pressure or using motor driven vibratory nozzle flaps orsolenoid-driven oscillating valves to promote mixing of the injectedprimary fluid with the surrounding secondary fluid.

The slotted nozzles described above may be fabricated from metals orother materials appropriate for the anticipated temperature and reactiveatmosphere in each application. When used in combustion applications,for example, the slotted nozzles can be made of type 304 or 316stainless steel.

The slotted nozzles described above may be used in combustion systemsfor the injection of fuel into combustion gases with high mixingefficiency. A sectional illustration of an exemplary burner system usingslotted nozzles is given in FIG. 8, which shows a central burner orflame holder surrounded by multiple slotted nozzles (which may bedefined as staging nozzles) for injecting secondary fuel. Central burneror flame holder 801 comprises outer pipe 803, concentric intermediatepipe 805, and inner concentric pipe 807. The interior of inner pipe 807and annular space 809 between outer pipe 803 and intermediate pipe 805are in flow communication with the interior of outer pipe 803. Annularspace 811 between inner pipe 807 and intermediate pipe 805 is connectedto and in flow communication with fuel inlet pipe 813. The centralburner is installed in furnace wall 814.

In the operation of this central burner, oxidant gas (typically air oroxygen-enriched air) 815 flows into the interior of outer pipe 803, aportion of this air flows through the interior of inner pipe 807, andthe remaining portion of this air flows through annular space 809.Primary fuel 816 flows through pipe 813 and through annular space 811,and is combusted initially in combustion zone 817 with air from innerpipe 807. Combustion gases from combustion zone 817 mix with additionalair in combustion zone 819. Combustion in this zone is typicallyextremely fuel-lean. A visible flame typically is formed in combustionzone 819 and in combustion zone 821 as combustion gases 823 enterfurnace interior 825.

A secondary fuel system comprises inlet pipe 827, manifold 829, and aplurality of secondary fuel injection pipes 831. The ends of thesecondary fuel injection pipes are fitted with slotted injection nozzles833 similar to those described above, for example, in FIGS. 1-3.Secondary fuel 835 flows through inlet pipe 827, manifold 829, andsecondary fuel injection pipes 831. Secondary fuel streams 837 fromnozzles 833 mix rapidly and combust with the oxidant-containingcombustion gases 823. Cooler combustion gases in furnace interior 825are rapidly entrained by secondary fuel streams 837 by the intensemixing action promoted by slotted nozzles 833, and the secondary fuel iscombusted with oxidant-containing combustion gases downstream of theexit of central burner 801. The primary fuel may be 5 to 30% of thetotal fuel flow rate (primary plus secondary) and the secondary fuel maybe 70 to 95% of the total fuel flow rate.

FIG. 9 is a plan view showing the discharge end of the exemplaryapparatus of FIG. 8. Concentric pipes 803, 805, and 807 enclose annularspaces 809 and 811 which are fitted with radial members or fins. Slottedsecondary fuel injection nozzles 833 (earlier described) may be disposedconcentrically around the central burner as shown. In this embodiment,the slot angles of the slotted injection nozzles are oriented to directinjected secondary fuel in diverging directions relative to the axis ofcentral burner 801.

Other types of slotted nozzles may be arrayed around the central burnerfor injecting secondary fuel. The nozzle bodies of these nozzles mayutilize one or more slots extending through the nozzle body from theinlet face to the outlet face, and each of these slots may becharacterized by a slot axis and a slot center plane as defined earlier.Each secondary fuel injector nozzle may have two or more slots and theslot axes of at least two slots may not be not parallel to each other.Alternatively, each secondary fuel injector nozzle may have two or moreslots and at least two of the slots may intersect each other.

EXAMPLE 1

A combustion test furnace utilizing the burner assembly of FIGS. 8 and 9was operated to compare the performance of the nozzles of FIGS. 1 and 4with a circular nozzle configuration illustrated in FIGS. 10A, 10B, and10C. These nozzles may be defined as staging nozzles which deliversecondary fuel to a second stage of combustion, wherein the fuel for thefirst stage of combustion is provided by fuel 815 via pipe 813 of FIG.8.

The test furnace was 6 ft by 6 ft in cross-section and 17 ft long, had aburner firing at one end, and had an outlet for the combustion productsat the other end. The outlet was connected to a stack fitted with adamper for furnace pressure control. The interior of the furnace waslined with high-temperature refractory and had water-cooled panels tosimulate furnace load. The test burner was fired in the range of 3 to 6MMBTU/hr using natural gas for the primary fuel and the secondary(staging) fuel. The flow rate of natural gas was varied between 3000SCFH and 6000 SCFH. The preferred flow of primary fuel was set at 500SCFH (8 to 16% of the total fuel) for 3 to 6 MMBTU/hr total firing rate.

The specific purposes of the tests were to determine fuel supplypressure requirements for optimum NO_(x) performance from various nozzleshapes at various firing rates and to determine optimum NO_(x) levelsfor these nozzles at different firing rates. The nozzle flow areas weregradually increased during various experiments for burners defined as“cross” and “zipper” nozzles (see below) to enable low fuel supplypressure operation and still obtain optimum NO_(x) emissions.

FIG. 10A is a top sectional view of circular nozzle 1001 using twoangled discharge holes 1003 and 1005 having circular cross sections. Thehole diameter was 0.11 inch and the radial angle α between the holes was15 degrees. FIG. 10B shows a side sectional view of the nozzle showingthe axial angle β between holes 1003 and 1005 and inlet flow axis 1007wherein the angle β was 7 degrees. FIG. 10C is a front view of thenozzle showing holes 1003 and 1005.

FIG. 11 shows views of the nozzle of FIGS. 5A, 5B, and 5C (describedherein as a “cross” nozzle) and includes notation for dimensions andslot angles. The height, H, and width, W, of exemplary slot 513 isdenoted in FIG. 11. FIG. 12 shows views of the nozzle of FIGS. 3A, 3B,3C, and 3D (described herein as a “zipper” nozzle) and includes notationfor dimensions and slot angles. The height, H, and width, W, ofexemplary slot 15 is denoted in FIG. 12. The dimensions and angles forthe nozzles used in the test furnace of this Example are given inTable 1. Typical ranges for these dimensions and angles are given inTable 2. TABLE 1 Dimensions for Nozzles Used in Test Furnace (Ro/R1)(H/Ro) Slot end Slot (α, α1, α2) (β) Fuel (W) radius to height to AxialRadial Staging (H) Slot center corner divergence divergence Nozzle SlotWidth, radius radius angle, angle, Type Height, (Inch) (Inch) ratioratio degrees degrees Cross 1/32 to 1 ¼ to 2 1.6 3.7 15 7 Nozzle (FIG.11) Zipper 1/32 to 1 ¼ to 2 1.6 3.7 15 7 Nozzle (FIG. 12)

TABLE 2 Typical Ranges for Nozzle Dimensions (Ro/R1) (H/Ro) Slot endSlot (α, α1, α2) (β) Secondary (W) radius to height to Axial Radial Fuel(H) Slot center corner divergence divergence Nozzle Slot Width, radiusradius angle, angle, Type Height, (Inch) (Inch) ratio ratio degreesdegrees Cross ( 1/32-1) (¼-2) (1-3) (2-6) (0-30) (0-30 Nozzle (FIG. 11)Zipper ( 1/32-1) (¼-2) (1-3) (2-6) (0-30) (0-30) Nozzle (FIG. 12)

The circular nozzle openings were drilled using standard twist drillswhereas the cross and zipper nozzles openings were machined usingElectro Discharge Machining (EDM). The main advantages of EDM are theability to machine complex nozzle shapes, incorporate compound injectionangles, provide higher dimensional accuracy, allow nozzle-to-nozzleconsistency, and maintaining closer tolerances. However, there arealternate manufacturing methods, such as high energy laser cutting, thatcan also produce equivalent nozzle hole quality as the EDM method.

The above dimensional ranges are valid for a variety of fuels, such asnatural gas, propane, refinery offgas, hydrogen PSA offgas, low BTUfuels, etc. The nozzles are optimally sized depending on fuelcomposition, flow rate (or firing rate) and supply pressure available atthe burner inlet. In Table 2, the dimensions, ratios and ranges areestimated for a 2 to 10 MM Btu/Hr burner firing rate. However, thesedimensions and ranges can be scaled up for higher firing rate burners(>10 MM Btu/Hr) using standard engineering practice of keeping similarflow velocity ranges.

The test furnace was operated using each of the circular, cross, andzipper nozzle types for secondary or staged firing to investigate theeffect of fuel pressure on firing rate and the effect of firing rate onNO_(x) emissions in the furnace flue gas. The primary and secondaryfuels were natural gas.

The test results are given in FIGS. 13 and 14. In FIG. 13, it is seenthat the measured range of firing rates was achieved at the lowest fuelpressures for the zipper nozzle of FIG. 1 (triangular data points), atintermediate fuel pressures for the star nozzle of FIG. 4 (square datapoints), and at the highest fuel pressures for the circular nozzle ofFIGS. 10A, B, and C (circular data points). The zipper nozzle of FIG. 1therefore is the preferred nozzle for use in secondary fuel staging inburner systems of the type illustrated in FIGS. 8 and 9, particularlyfor fuel available only at the lowest pressures.

In FIG. 14, which is a plot of the NO_(x) concentration in the testfurnace flue gas discharge as a function of firing rate, it is seen thatthe lowest NO_(x) concentrations were measured for the zipper nozzle ofFIG. 1 (triangular data points). Higher NO_(x) concentrations weremeasured for the star nozzle of FIG. 4 (square data points) and thehighest NO_(x) concentrations were measured for the circular nozzle ofFIGS. 10A, B, and C (circular data points). These results indicate thatthe zipper nozzle operates at very low NO_(x) emission levels andperforms significantly better than the star and circular nozzles.

The cross- and zipper-shaped nozzles of the present invention operatedat lower nozzle tip operating temperatures than the circular nozzle ofFIGS. 10A, B, and C. It was observed during the laboratory experimentsthat the overall fuel supply pressure for the circular nozzle requiredincreases to account for a lower nozzle flow coefficient as the nozzleoperating temperatures increased above ambient. This was partly due tolocalized heating of the circular nozzle tips due to the fuel gasexpansion effect at higher operating temperature. For this reason, thecircular tip fuel supply pressure data required adjustment for higheroperating temperature. The flow correction factor from ambient to theoperating tip temperature (˜450° F.) was about 0.58 for the circularnozzle, and this resulted in 42% less fuel flow due to the nozzle tiptemperature.

In contrast, the zipper fuel nozzles have a relatively large exit flowarea, and the nozzle tip was actively cooled by the exiting fuel gasstream. Unlike the circular nozzle, which has a relatively largestagnation region at the tip, the zipper nozzle has a much higher activecooling zone due to the number of narrow intersecting slots in thenozzle tip. The zipper nozzle required a smaller flow correction factorof 0.77 from ambient to operating the tip temperature (˜250° F.), andthus required an approximately 33% lower fuel flow correction factor.This is significantly lower than the 450° F. temperature fuel flowcorrection factor required for the circular nozzles. Overall, thecircular nozzles required a fuel supply pressure 5 times higher than thezipper nozzle for the same burner firing rate, probably due torelatively poor entrainment efficiency and higher operating tiptemperature of the circular nozzle. The advantages of lower operatingtip temperatures for the zipper or cross nozzles includes (a) reducedtendency to coke when using higher carbon content fuels, (b) the abilityto use smaller fuel flow rates or higher heating value fuels, and (c)the ability to use less expensive material for the nozzle material.Because of the operating tip temperature differences, type 304 or 310stainless steel can be used for the zipper or cross nozzles whileHastelloy®, Inconel®, or other high-temperature alloys may be requiredfor the circular nozzles.

Thermal cracking is a concern in many refinery furnace applications inwhich the fuel gas contains C₁ to C₄ hydrocarbons. The cracking of theheavier hydrocarbons, which occurs much more readily at the higheroperating temperatures of circular nozzles, produces carbon that canplug burner nozzles, cause overheating of burner parts, reduce burnerproductivity, and result in poor thermal efficiency. The lower operatingtemperatures of the zipper and cross nozzles thus allowsmaintenance-free operation, and this is an advantage in the applicationof these burners in refinery furnace operations.

The slotted nozzles described above may be used in a combustion devicefor injecting a primary fluid where the primary fluid is a diluted fuelmixture.

As shown in FIG. 15, for example, a combustion device 2 comprises aconduit portion 21 for conveying a fuel, and a conduit portion 23 forconveying a fluid which is different from the fuel. Conduit portion 21conveys the fuel separately from the fluid and conduit portion 23conveys fluid separately from the fuel. A mixing conduit 25 is in fluidcommunication with the conduit portion 21 and in fluid communicationwith the conduit portion 23. The fuel and the fluid come together andmix in the mixing conduit 25 to form a diluted fuel mixture. The fueland the fluid need not be completely mixed or homogenized in mixingconduit 25. The fluid to fuel ratio may be in the range of 1:20 to 20:1,or 1:10 to 10:1, or 1:4 to 4:1. An amount of fluid useful for reducingNOx emissions may be determined without undue experimentation.

A conduit is defined as any structure for containing flow, for example,pipes, tubing, ducts, and the like.

A nozzle is in fluid communication with the mixing conduit 25 forpassing the diluted fuel mixture therethrough. The nozzle comprises anozzle body 3 and one or more slots extending through the nozzle body 3.FIG. 15, for example, shows five slots 7, 9, 11, 13, and 15. The nozzlebody 3 has an inlet face 203, an outlet face 17, and an inlet flow axis201 passing through the inlet face 203 and the outlet face 17. The oneor more slots extend through the nozzle body from the inlet face 203 tothe outlet face 17 and each slot has a slot axis. Slot axes may bestraight as shown in FIG. 15 or curved (not shown).

As shown in FIG. 15, conduit portion 21 is disposed within conduitportion 23. A jet ejector or jet pump effect may be created when thefuel in conduit portion 21 has a greater velocity than the fluid inconduit portion 23. The conduit portion 21 has a conduit portion outlet27, which has an equivalent diameter. The equivalent diameter of aconduit portion outlet is the diameter of a circle having the same areaas the conduit portion outlet. In case the conduit portion outlet iscircular, the equivalent diameter of the conduit portion is the innerdiameter of the conduit portion outlet. The length of the mixing conduit25 for mixing the fuel and the fluid may be 2 to 20 times the equivalentdiameter of the conduit portion 21 as indicated by L₂₅ in FIG. 15. Thelength of the mixing conduit 25 for mixing the fuel and the fluid may be0.0625 inches (1.59 mm) to 1 inch (25 mm), and may be selected based onanticipated operating conditions.

Alternatively, as shown in FIG. 16, conduit portion 23 for conveying thefluid may be disposed within conduit portion 21 for conveying the fuel.A jet ejector or jet pump effect may be effected when the fluid inconduit portion 23 has a greater velocity than the fuel in conduitportion 21. The length of the mixing conduit 25 for mixing the fuel andthe fluid may be 2 to 20 times the equivalent diameter of the conduitportion 23.

Alternatively, as shown in FIG. 17, conduit portion 21 for conveying thefuel is not disposed within conduit portion 23 for conveying the fluid,and conduit portion 23 is not disposed within conduit portion 21. Incase neither conduit portion 21 or conduit portion 23 is disposed in theother, the length of the mixing conduit 25 for mixing is the distance,in the inlet flow axis direction, between the inner face 203 of thenozzle body 3 and the furthest intersection point 22 between the wall ofconduit portion 25 and the wall of conduit portion 23 as indicated byL₂₅ in FIG. 17.

At least one of the conduit portion 21 and the conduit portion 23 maycontain a swirling means (not shown). A swirling means may be vanes, orchannels, angled to generate swirl as known in the art.

The combustion device described above may be used in a combustion methodfor injecting a primary fluid where the primary fluid is a diluted fuelmixture.

The combustion method comprises mixing a first substantially gaseousfuel and a fluid in a conduit thereby forming a diluted fuel mixture andpassing the diluted fuel mixture through a nozzle. The firstsubstantially gaseous fuel has a first fuel index and the fluid has asecond fuel index which is different than the first fuel index. Thenozzle comprises a nozzle body having an inlet face, an outlet face, andan inlet flow axis passing through the inlet and outlet faces; and oneor more slots extending through the nozzle body from the inlet face tothe outlet face. Each slot has a slot axis.

Mixing of the first substantially gaseous fuel and the fluid may beachieved in any of a number of ways, for example, as illustrated inFIGS. 15-17.

The fluid may be another fuel which is different than the firstsubstantially gaseous fuel or it may be a substantially non-reactinggas.

As used herein, the term “fuel index” (FI) is defined as the weightedsum of the fuel carbon atom number, the weights being the component molefractions: FI=ΣC_(i)x_(i), where C_(i) and x_(i) are the number ofcarbon atoms and the mole fraction of component i, respectively.Molecular H₂ is assigned a carbon number of 1.3 for reasons discussedbelow.

Fuel indices of a number of fuels and other fluids are listed in Table3. Generally, a fuel with a higher fuel index cracks more easily andproduces more NOx through the prompt NOx mechanism. H₂ is a special casein this definition. Although H₂ does not have any carbon atoms, it iswell-known that H₂ addition in natural gas increases NOx emissions. Theliterature suggests that about a 30% higher NOx emission occurs for pureH₂ flames as compared to methane flames. The increased NOx emission fromH₂ flames may be attributed to the thermal NOx mechanism due to higherflame temperatures. Since the fuel index is used as an indicator for NOxemission herein, a value of 1.3 is assigned to H₂ to be consistent withits NOx emission potential.

The first substantially gaseous fuel may be mixed with a fluid whereinthe fluid is a second substantially gaseous fuel. For example, the firstsubstantially gaseous fuel may be refinery offgas at a high supplypressure that may contain a blend of hydrogen and higher carbon tohydrogen ratio fuels (e.g. ethane, propane, butane, olefins, etc.) Thesecond substantially gaseous fuel may be a lower pressure fuel gashaving a lower fuel index (e.g. hydrogen, syngas, natural gas, or a lowBTU fuel blend). The dilution of the refinery offgas may help alleviatemaintenance problems due to soot build-up on the burner fuel tips causedby thermal cracking of the refinery offgas. The dilution of the refineryoffgas may also decrease NOx emissions. TABLE 3 Fuel Indices forSelected Fuels and Other Fluids Fuels or Other Fluids Fuel Index H₂ 1.3H₂O 0 CO₂ 0 CO 1 N₂ 0 CH₄ 1 C₃H₈ 3 Refinery Offgas (1) 1.434 PSA offgas(2) 0.57 PSA offgas (3) 0.64 Natural gas (4) 1.08 Natural gas (5) 1.14(1) Refinery Offgas: 18% H₂, 44% CH₄, 38% C₂H₂.(2) PSA offgas: 30% H₂, 18% CH₄, 52% CO₂.(3) PSA offgas: 30% H₂, 15% CH₄, 45% CO₂, 10% CO.(4) Natural gas: 91% CH₄, 4% C₂H₆, 3% C₃H₈, 1% N₂, 1% CO₂.(5) Natural gas: 84% CH₄, 12% C₂H₆, 2% C₃H₈, 2% N₂.

With reference to FIG. 15, a high-pressure (for example, in the range of2 to 50 psig, (115 to 445 kPa)) fuel, which may be refinery offgas, ispassed through conduit portion 21, while a second substantially gaseousfuel, such as natural gas, syngas, process gas, PSA offgas, etc. whichmay have a lower pressure (for example, in the range of 0.1 to 3 psig,(102 to 122 kPa)) is passed through conduit portion 23. The secondsubstantially gaseous fuel passes through the annular space defined byconduit 21 and conduit 23. PSA offgas is a byproduct from hydrogen PSAadsorbent beds. The velocity of the fuel may be 900 to 1400 feet/second(275 to 425 m/s) and may be sonic velocity i.e. choked flow. Thevelocity of the second substantially gaseous fuel may be 100 to 900feet/sec (25 to 275 m/s), depending on the available supply pressure.

As shown in Table 3, a representative refinery offgas has a fuel indexof 1.434, while a representative PSA offgas has a fuel index of 0.57 andone representative natural gas has a fuel index of 1.08 and anotherrepresentative natural gas has a fuel index of 1.14. The fuel index ofthe second substantially gaseous fuel is less than the fuel index of thefirst substantially gaseous fuel by at least 0.1, or by at least 0.25,or by at least 0.75.

As shown in FIG. 15, the first substantially gaseous fuel (e.g. refineryoffgas) and the second substantially gaseous fuel mix in the mixingconduit 25 thereby forming a diluted fuel mixture. The diluted fuelmixture is passed through a nozzle, the nozzle comprising a nozzle body3 having an inlet face 203, an outlet face 17, and an inlet flow axis201 passing through the inlet face 203 and the outlet face 17, and oneor more slots, for example slots 7, 9, 11, 13, and 15 extending throughthe nozzle body from the inlet face 203 to the outlet face 17.

The combustion method may be used in steam methane reformers where thefirst substantially gaseous fuel may be natural gas or refinery offgas.The second substantially gaseous fuel may be PSA offgas. The natural gasor refinery offgas may account for between 10% and 30% of the totalenergy for typical reformers having PSA for hydrogen separation.Hydrogen PSA offgas accounts for the remaining energy.

EXAMPLE 2

In laboratory testing using the combustion device and combustion methodin the test furnace described in Example 1, a burner had 10 fuel lancesevenly distributed around a circle of 18″ diameter. Of the 10 fuellances, two fuel lances were combustion devices described above with amixing conduit, positioned opposite each other in the circle. Eight ofthe fuel lances had the geometry of the nozzle without a mixing conduitlike that shown in FIG. 2 and two of the fuel lances had the geometry ofthe nozzle with a mixing conduit like that shown in FIG. 15.

The burner was rated at a firing rate of 8 MMBtu/hr utilizing 644° F.preheated air. In this example, the fluid for diluting the fuel was alsoa fuel. The fuel was a simulated refinery offgas and contained 18%hydrogen, 44% local natural gas, and 38% ethylene. The fuel index ofthis simulated refinery offgas was about 1.43. The other fuel simulatedPSA offgas and contained 52% carbon dioxide, 18% local natural gas, and30% hydrogen. The fuel index of this simulated PSA offgas was about0.57.

For each of the experiments, the burner was fired at a rate of about 8MMBtu/h with 70% of the total energy input to the laboratory furnacefrom simulated PSA offgas and 30% of the total energy input to thelaboratory furnace from the simulated refinery offgas.

Referring to the arrangement illustrated in corresponding FIGS. 12 and15, the simulated refinery offgas was injected in a conduit portion 21made of standard tubing having ⅜″ (9.525 mm) diameter and 0.035″ (0.89mm) wall thickness, which was placed concentrically in a conduit portion23 made of pipe of ¾″ (19 mm) sch. 40. Zipper nozzles, described above,were used having dimensions given in Table 1. The zipper tip was sizedfor 0.51″ (13 mm) equivalent diameter and, as shown in FIGS. 12 and 15,had four “horizontal” slots and one “vertical” slot. The slots are“horizontal” and “vertical” in the figures, but may have variousorientations when mounted in the furnace. The divergence angles (α1 andα2) for the vertical slots were about 18° and 6°, respectively. Thedivergence angle, β, was about 7°.

In one experiment, simulated PSA offgas was distributed between theeight nozzles without a mixing conduit, and simulated refinery offgaswas distributed between the two nozzles with a mixing conduit. No fluidwas mixed with the simulated refinery offgas to form a diluted fuelmixture in this case. NOx emissions were measured at 25 to 30 ppmv.

In another experiment, simulated PSA offgas was mixed with the simulatedrefinery offgas in the two nozzles with a mixing conduit. NOx emissionswere measured at less than 15 ppmv. In addition, when simulated PSAoffgas was mixed with simulated refinery offgas, more uniform heattransfer to the load in the furnace was observed based on thermocouplereadings.

Visually, the flame without mixing the simulated PSA offgas with thesimulated refinery offgas was more visible than the flame with mixing.Even as the composition of the simulated refinery offgas was adjusted tohave as much as 50% butane, flameless combustion was observed when thesimulated refinery offgas was diluted with the simulated PSA offgas.

EXAMPLE 3

Diluted fuel mixtures may be formed by mixing generally nonreactinggases, such as steam, carbon dioxide, flue gas, nitrogen, or other inertgases with a fuel.

In another experiment, all of the fuel lances in Example 2 were fittedwith a mixing conduit and each of the fuel lances had zipper nozzles asdescribed above. Referring to the arrangement illustrated incorresponding FIGS. 12 and 15, in one experiment nitrogen was introducedthrough conduit portion 21 and natural gas was introduced throughconduit portion 23. In another experiment, no nitrogen was introduced.

The burner was operated at a firing rate of about 5 MMBtu/h usingambient combustion air. The average furnace operating temperature wasabout 1600° F. and the exhaust gas temperature was about 2000° F. Forthe case with nitrogen dilution, nitrogen was introduced to the burnerwith a flow rate of about 10% of the total flow on a weight basis. Forthe case without nitrogen dilution, NOx was measured at about 10 ppm(corrected at 3% oxygen) while for the case with nitrogen dilution, NOxwas measured at about 7 ppm (corrected at 3% oxygen).

EXAMPLE 4

Conventionally, in a reformer, steam may be introduced with thecombustion air at a rate of 0.25 to 0.5 Ibm steam/Ibm fuel to lower NOxemissions.

Referring to the arrangement illustrated in corresponding FIGS. 12 and16, high pressure (30 to 100 psig, 300 kPa to 800 kPa) steam may beintroduced through conduit portion 23 at about 900 to 1400 feet/sec (275to 425 m/s) and a fuel gas may be introduced through the annular spacedefined between conduit portion 23 and conduit portion 21. The highvelocity steam jet exiting conduit portion 23 may entrain the fuel gasand mix in the mixing conduit 25. The resulting diluted fuel mixture isthen passed through the nozzle at a velocity of about 600 to 1400 feet/s(175 to 425 m/s). The diluted fuel mixture may then entrain furnacegases to form a furnace gas entrained fuel mixture. The furnace gasentrained fuel mixture may then combust with oxidant gas which has beenintroduced to the furnace. As a result of the steam dilution and furnacegas dilution, peak flame temperature may be reduced below peak flametemperatures obtained without the steam dilution, resulting in very lowNOx emissions.

Table 4 provides steam consumption estimates for a large steam methanereformer. As shown in Table 4, due to the combustion method of fuelstaging with steam, the amount of steam required may be much lower (0.02to 0.05 Ibm steam per Ibm fuel, (0.02 to 0.05 kg steam per kg fuel))than conventional steam injection (0.25 to 0.5 Ibm steam per Ibm fuel,(0.25 to 0.5 kg steam per kg fuel)).

In addition to reduced NOx emissions, other benefits may includeimproved nozzle tip cooling, and reduced tendency to form soot even withhigher carbon content fuels. If nozzle tips operate at lowertemperatures, lower cost nozzle materials may be used, for example 304stainless steel or 310 stainless steel.

Thermal cracking (soot formation) is a concern for many refineryfurnaces where fuel compositions contain hydrocarbons ranging from C1 toC4. Soot is found to plug burner nozzles and create overheating ofburner parts, reduced productivity and poor thermal efficiency. TABLE 4Steam Consumption Economics with Steam Dilution (units) (quantity)(quantity) Steam injection rate lbm steam/lbm fuel 0.02 0.05 Firing rateMMBtu/h, lhv 850 850 Fuel heating value Btu/scf, lhv 1000 1000 Fuel cost$/MMBtu, lhv 6 6 Fuel molecular weight 18 18 Steam needed Lb/hr 806 2016MMscfd 0.408 1.02 Energy require to Btu/scf 57.1 57.1 generate At 100psia Btu/lb 1203.2 1203.2 and 400° F. from water at 60° F. Steam cost$/day 140 349 $/year 50,992 127,480

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

1. A combustion method comprising: mixing a first substantially gaseousfuel having a first fuel index and a fluid having a second fuel indexwhich is different than the first fuel index in a conduit therebyforming a diluted fuel mixture; and passing the diluted fuel mixturethrough a nozzle, the nozzle comprising: a nozzle body having an inletface, an outlet face, and an inlet flow axis passing through the inletface and the outlet face; and one or more slots extending through thenozzle body from the inlet face to the outlet face, each slot having aslot axis.
 2. The combustion method of claim 1 wherein the nozzlecomprises two or more slots extending through the nozzle body from theinlet face to the outlet face.
 3. The combustion method of claim 1wherein the second fuel index is less than the first fuel index by atleast 0.1.
 4. The combustion method of claim 1 wherein the fluid is asecond substantially gaseous fuel.
 5. The combustion method of claim 1wherein the fluid is selected from the group consisting of steam, carbonmonoxide, carbon dioxide, nitrogen, argon, helium, xenon, krypton andmixtures thereof.
 6. The combustion method of claim 1 wherein the fluidcomprises flue gas.
 7. The combustion method of claim 1 wherein thefluid comprises hydrogen PSA offgas.
 8. The combustion method of claim 1wherein the first substantially gaseous fuel comprises refinery offgas,natural gas, or hydrogen PSA offgas.
 9. The combustion method of claim 1wherein the first substantially gaseous fuel comprises methane orpropane.
 10. The combustion method of claim 1 further comprising:introducing an oxidant gas; and combusting at least a portion of thediluted fuel mixture with at least a portion of the oxidant gas.
 11. Thecombustion method of claim 1 further comprising: entraining a furnacegas in at least a portion of the diluted fuel mixture in a furnacethereby forming a furnace gas entrained fuel mixture; introducing anoxidant gas; and combusting at least a portion of the furnace gasentrained fuel mixture with at least a portion of the oxidant gas. 12.The combustion method of claim 1 further comprising: swirling at leastone of the first substantially gaseous fuel and the fluid prior tomixing the first substantially gaseous fuel and the fluid.
 13. Thecombustion method of claim 1 wherein the diluted fuel mixture has aresidence time in the conduit and the residence time is 0.1 to 10milliseconds.
 14. A combustion device comprising: a first conduitportion for conveying a fuel; a second conduit portion for conveying afluid which is different from the fuel; a mixing conduit in fluidcommunication with the first conduit portion and in fluid communicationwith the second conduit portion for mixing the fuel and the fluid toform a diluted fuel mixture; and a nozzle in fluid communication withthe mixing conduit for passing the diluted fuel mixture therethrough,the nozzle comprising: a nozzle body having an inlet face, an outletface, and an inlet flow axis passing through the inlet face and theoutlet face; and one or more slots extending through the nozzle bodyfrom the inlet face to the outlet face, each slot having a slot axis.15. The combustion device of claim 14 wherein the nozzle comprises twoor more slots extending through the nozzle body from the inlet face tothe outlet face.
 16. The combustion device of claim 15 wherein the slotaxis of at least one of the slots is not parallel to the inlet flow axisof the nozzle body.
 17. The combustion device of claim 15 wherein theslot axes of at least two slots are not parallel to each other.
 18. Thecombustion device of claim 15 wherein at least two of the slotsintersect each other.
 19. The combustion device of claim 15 wherein noneof the slots intersect.
 20. The combustion device of claim 15 havingthree or more slots wherein a first slot of the three or more slotsintersects with a second slot of the three or more slots and a thirdslot of the three or more slots.
 21. The combustion device of claim 15having four or more slots wherein a first slot and a second slotintersect each other and a third slot and a fourth slot intersect eachother.
 22. The combustion device of claim 14, wherein the first conduitportion is disposed within the second conduit portion.
 23. Thecombustion device of claim 22, wherein the first conduit portion has anoutlet, the outlet of the first conduit portion has an equivalentdiameter, and the mixing conduit has a length in a range of 2 to 20times the equivalent diameter of the outlet of the first conduitportion.
 24. The combustion device of claim 14, wherein the secondconduit portion is disposed within the first conduit portion.
 25. Thecombustion device of claim 24, wherein the second conduit portion has anoutlet, the outlet of the second conduit portion has an equivalentdiameter, and the mixing conduit has a length in a range of 2 to 20times the equivalent diameter of the outlet of the second conduitportion.
 26. The combustion device of claim 14, wherein the firstconduit portion is not disposed within the second conduit portion andthe second conduit portion is not disposed within the first conduitportion.
 27. The combustion device of claim 14, further comprising aswirling means in at least one of the first conduit portion and thesecond conduit portion.